Wind turbines use wind energy to generate electricity. A conventional wind turbine includes a rotor mounted on a tower. The rotor may turn up to about 60 rpm in a steady wind of about 20 mph and connect to an alternator/generator through a speed increasing transmission. Typical generators include synchronous or asynchronous generators and require a constant input shaft speed of about 1200 or 1800 rpm, depending on the type of generator, to produce quality power. Although variable speed generators are available, the power output of a variable speed generator must be conditioned before it can be fed into a power grid.
It is a continuing problem with wind-driven turbines to provide a cost-effective method of smoothing the torque generated by the rotor so as to reduce torque fluctuations in the drivetrain to the generator. It is known to use complex transmissions to attempt to provide constant speed input to a generator. It is also known to use variable pitch rotors and braking mechanisms in an effort to operate at a constant rotor speed. However, such transmissions, variable pitch rotors and braking mechanisms are expensive, prone to mechanical breakdown and introduce a significant source of mechanical inefficiency to the wind turbine operation.
Wind turbines using an open loop hydraulic system in place of mechanical transmissions are also known. However, conventional hydraulic pumps require input speed of a minimum of about 300 to 500 rpm to produce usable hydraulic pressure. As a result, a mechanical speed increaser is still required between the rotor and the hydraulic pump. All known hydraulic systems for wind turbines utilize an open loop system. For example, in U.S. Pat. No. 4,503,673, a positive displacement hydraulic pump connected to a variable displacement hydraulic motor is disclosed. In this system, similar to other open loop systems, the hydraulic pump is elevated on the tower but the hydraulic motor, hydraulic fluid reservoir and generator are on the ground. Although it is generally advantageous to reduce the tower load, this arrangement necessitates long hydraulic fluid hoses to and from the hydraulic pump, which is disadvantageous.
A closed-loop hydraulic system would permit all of the components, including the generator to be tower mounted in the nacelle. However, a closed-loop system has not been attempted because of the difficulty in dealing with overspeed situations. In a closed loop system, hydraulic resistance to the increased torque would result in intolerable heat buildup in the system, if fluid pressure is not controlled.
In an open-loop hydraulic system, when the rotor is driven at high speed, excess hydraulic pressure may be diverted by “dumping” pressure to maintain a constant generator speed. This energy dissipation generates tremendous amounts of heat and active cooling or heat exchanging is necessary. For example, in U.S. Pat. No. 4,149,092, a hydraulic system for water and wind driven turbines is disclosed which includes a shunt-connected energy dissipator. In response to high pressure caused by high wind and rotor speeds, the displacement of the hydraulic motor decreases, further increasing system pressure. As a result, the hydraulic fluid is diverted into the energy dissipator. The dissipator converts hydraulic energy into heat which is removed by a heat exchanger.
In U.S. Pat. No. 4,503,673, referred to above, complex hydraulic controls are used to feather the rotor propeller blades in order to deal with excess pressure in the hydraulic circuit.
Therefore, there is a need in the art for a wind turbine system utilizing a closed loop hydrostatic transmission which mitigates the difficulties of the prior art.